Some bloggers like Ming Thein reported the EM-1 could perform at 1 stop ISO advantage over the EM-5. What they forgot to mention is that this is for JPEG files ONLY.

Indeed, the EM-1 has a visibly improved JPEG engine over the last generation and the competition, and it does yield nicer images in high ISO settings. However this does not apply to RAW at all. The EM-1 is tied with the OM-D EM-5 and the GX7 for RAW captures. Just look at DPreview's own test charts. The EPL-5 even seems to beat it in high ISO detail.

Let's keep it honest, guys. the EM1 was massively hyped by review after review comparing it to Full Frame cameras. But at the end of the day, it's the same Sony sensor we know and are used to, same DR, same ISO sensitivity, sure it also has PDAF, no AA filter and better JPEGs but that's it.

Actually Ming Thein does claim that the E-M1 has noticeably better IQ even for RAW.

Now that ACR 8.2 has support for the E-M1′s raw files, I’ve gone back through the archive, reprocessed a few and had another look at the overall file quality. Fundamentally, the good news is that none of the observations really change: underlying sensor quality remains excellent, and a small but distinct step up on the E-M5′s sensor. There is a definite gain in edge acuity and ability to resolve fine detail; the high ISO noise improvement is not as pronounced as with the JPEGs, but the red channel especially shows clear signs of improvement in luminance noise, color accuracy and tonal separation. I would estimate the advantage of the newer sensor to be about half a stop up to ISO 1600

I am somewhat skeptical myself but we shall see. Early reviews tend to be pretty enthusiastic with a lot of cameras.